Answer:
The independent variable is the type of juice being used (lemon juice, vinegar, or grape juice), while the dependent variable is the strength of the juice.
It's important to note that the term "strength" is a bit ambiguous and could have different meanings depending on the context. If you're comparing the acidity of the juices, for example, you might measure their pH levels to determine which is the strongest acid. If you're comparing their ability to clean a stain, you might measure the amount of stain removal to determine which is the strongest. So, the specific operational definition of "strength" would need to be determined in order to carry out the experiment.